Short answer: eat. A lot. Not to the point that you constantly feel full and sluggish, but learn eat even when you're not that hungry. Chicken breast, lean beef, penut butter, wheat bread, milk, and a lot of other foods are great for this.

And start lifting heavy occasionally. Whatever you can safely lift for no more than 6-8 reps, maybe less, should be good. And start squating and deadlifting if you aren't already. But like others have said; don't just jump into heavy lifting without a little help and/or guidance.

It worked for me and I'm currently a lean 5'6 and 170. But there are a ton of other methods that may be more effective and/or easier on your body though, so just keep experimenting.

While I can understand your point and view, and agree with it, for me, it's unfortunately neglectable at this point. =\

Why? Because my goal isn't to really build up that much muscle mass, primarily my goal is just to gain more weight, while I understand that someone who is already that well up in weight may think it's very important to check their muscle mass, for me, my primary goal is still just to gain weight.

I'm also gaining some muscle along the way, but I view that more as a bonus than part of my main objective, I also do some excercising but that's mostly to get a more balanced body composition, rather than all of the fat just going to the 'bad places'.

Eh, ****, whenever I write a long message it turns into total crap that I feel like editing, but my point is: checking your fat-level is cool and all, but it's too specialized (and unneccesary) for me at this point.

Dude, I've weighed 220 pounds as a fatty, and now I weigh 220 pounds and am relatively lean. Let me tell you, my current self could absolutely clobber the living crap out of my old self. Muscle mass as a bonus? It should be your main objective, or you will just be a fat weakling instead of a skinny weakling.

I lift 3 times a week, eat at least 5 times a day, drink plenty of protein shakes and get plenty of good rest. I'm not really looking to get any bigger, so I just try to maintain a comfortably satiated feeling, but when I was actively bulking, I just drank low-fat milk every chance I got. The most effective weight-gaining exercise for me, in my opinion, was the squat. A hell of a lot of my weight is now in my thighs (and lower back).

In other words, your question was answered on page 1.

EDIT: I just noticed someone managed to convince you on the last page, anyway hopefully what I said will give you added motivation to shoot for that rainbow or something.